An interconnect circuit board is a physical realization of electronic circuits or subsystems made from a number of extremely small circuit elements that are electrically and mechanically interconnected. It is frequently desirable to combine these diverse type electronic components in an arrangement so that they can be physically isolated and mounted adjacent to one another in a single compact package and electrically connected to each other and/or to common connections extending from the package.
Complex electronic circuits generally require that the circuit be constructed of several layers of conductors separated by insulating dielectric layers. The conductive layers are interconnected between levels by electrically conductive pathways, called vias, through a dielectric layer. Such a multilayer structure allows a circuit to be more compact.
Typically, a LTCC tape is formed by casting a slurry of inorganic solids, organic solids and a fugitive solvent on a removable polymeric film. The slurry consists of glass powder(s) and ceramic oxide filler materials and an organic based resin-solvent system (medium) formulated and processed to a fluid containing dispersed, suspended solids. The tape is made by coating the surface of a removable polymeric film with the slurry so as to form a uniform thickness and width of coating.
To interconnect the electronic functions, precious metal-based thick film pastes printed over the tapes, conductor lines are connected with via-fill conductors, laminated and fired at appropriate temperature to obtain an electronic device. Circuits with silver-based conductors are cost-effective compared to gold-based circuits but they are less reliable particularly if they are exposed to atmospheric humidity.
During the exposure to atmospheric conditions, surface silver conductor will tarnish (oxidation of silver is common phenomenon during the exposure to atmospheric conditions) and circuit functions will fail. To minimize such oxidation phenomenon, coating of surface silver conductor with nickel-gold is a common practice. However base nickel plating which is necessary before plating of gold, generally “lifts” the silver conductors from the LTCC unlike other substrates such as alumina, printed wiring board and copper etc. Such silver conductor pad lifts, no matter how small degree, will degrade the final functional properties such as wire-bonding strength, long term reliability etc of the finished LTCC circuits. In order to minimize bond lifts, plating conditions, physical properties of the plated devices should select in such way to give good wire-bonding conditions and reliability. The major plating conditions that need to be adjusted are: pH and temperature of the plating bath appropriate to the composition of the LTCC and silver conductor; plating thickness of nickel and gold; and the ratio of nickel to gold thickness; Pd-base plating over silver, if needed etc. Several open literature on electroless plating are available, but none are specific to particular chemistry of the LTCC which is one on the most important parameter to determine the appropriate plating conditions and thickness of nickel to gold. (“ElectrolessPlating-Fundamentals & Applications” 1990, edited by G. O. Mallory & J. B. Hajdu, Williams Andrew Publishing/Noyes, Chapter 1, The Fundamental Aspects of Electroless Nickel Planting by Glenn O. Mallory.) D. Gudeczaukas, Uyemura International corporation, Southington CT published several publication on electroless gold plating techniques, particularly one on LTCC in Ceramic Industry, December 2007, p. 13. Several patents describe nickel-gold plating conditions and applications: for example U.S. Pat. Nos. 6,156,218 and 6,362,089 on copper substrates; U.S. Pat. No. 6,548,327 on aluminum etc. The general techniques described in the literature and industry-wide accepted methods are electroless plating of nickel-gold or nickel-palladium-gold under acidic solution pHs at 77-90° C. The LTCC circuits based on Dupont Green tape systems that are described US 2007/011187 will not plate wire-bondable gold under the industry wide accepted plating conditions due to the conductor-substrate bonding failure of the plated parts, irrespective of variation in silver conductor composition. Furthermore, no other LTCC circuits plated with nickel-gold and wire-bondable with long-term adhesion/reliability are presently available in the market place.
The present invention overcomes problems associated with prior art thick film platable silver conductor compositions for use in LTCC-based nickel-gold plated electronic devices and applications. In particular, the present invention provides thick film compositions and plating conditions required to obtain wire-bondable LTCC structures, which provide superior long-term reliability.